An alkylation unit is commonly used in modem petroleum refining to react isobutane with a light olefins source (e.g., at least one of propylene and butylene) in the presence of a strong acid catalyst to form a branched C7 or C8 alkane generally referred to as alkylate. Alkylate is a desirable motor gasoline blending component because it has a low vapor pressure, a high octane content, and a low sulfur content.
One of the reactions taking place in the alkylation unit involves connecting light hydrocarbon molecules, e.g., connecting light olefin or isoolefin to isoalkane or isoalkene, such as connecting a C3 olefin to a C4 isoolefin or a C4 isoalkane. Conventionally, the C3 olefin is obtained from refining processes as a component of a gas mixture called refinery grade propylene (“RGP”). RGP comprises primarily C3 olefin and propane. Since propane does not react in the alkylation unit, the alkylation process includes means for separating propane from the alkylate downstream of the alkylation unit.
The alkylation process does not require pure propylene or pure iso-C4 hydrocarbon. Thus, it has heretofore not been considered desirable to concentrate the olefins source, e.g., by removing alkanes, before feeding the olefins to the alkylation unit. Rather, light olefins source streams are typically produced by a refinery in terms of “carbon-number purity”. That is, C3 hydrocarbons are collected in one stream and C4 hydrocarbons are collected in a separate stream. A C3 light olefins source stream can comprise about 60 wt. % propylene, with the remainder comprising mostly propane. An iso-C4 hydrocarbon source stream can comprise ≥50 wt. % iso-C4 hydrocarbon, with ≥50 wt. % of the remainder comprising butane, typically normal butane. One conventional process for producing alkylate is disclosed in Alkylation of Isobutane with Propylene, E. C. Oden and W. J. Burch, Industrial and Engineering Chemistry 41, 11, 2524-2529 (1949). The reference discloses producing alkylate by reacting olefin (propylene and/or butylene) with isobutane in the presence of a sulfuric acid catalyst. Unreacted vapors separated from the reactor's effluent are compressed in a refrigeration compressor and then recycled to the alkylation reactor. Isobutane and butane are separated from the liquid portion of reactor effluent. Light and heavy alkylates are conducted away. Propane present in the feed is recovered in a depropanizer stage. As disclosed in the reference, the depropanizer is integrated with both the refrigeration and alkylate-recovery stages.
It has been observed that the capacity of the alkylation unit can be limited by the amount of propane in the RGP feed. If the amount of propane was lessened and the amount of propylene was increased, the alkylation unit could make more alkylate. Thus, propane is a diluent that bottlenecks the alkylation unit. It is desired to at least partially overcome this bottleneck.